1. Field of the Invention
The present invention relates to a power amplifier apparatus.
2. Description of the Related Art
In some audio power amplifier apparatuses, power amplification is performed by switching. Such a switching-type power amplifier apparatus is configured, for example, as shown in FIG. 1.
In FIG. 1, a broadcast wave, such as a frequency modulation (FM) broadcast wave, is received by an antenna 11 and is supplied to a receiving circuit 12 where an analog audio signal of a target broadcast is extracted. The analog audio signal is then supplied to a switching-type power amplifier apparatus 20.
In the power amplifier apparatus 20, the audio signal output from the receiving circuit 12 is supplied to a converter circuit 21, where it is converted into a digital audio signal. Simultaneously, the digital audio signal is converted into a pulse modulation signal, for example, a pulse width modulation (PWM) signal P21, in which the integrated value of one sample is equivalent to the level of the original analog audio signal. The carrier frequency of the PWM signal P21 is, in general, from a few hundred of kilohertz to a few megahertz.
The PWM signal P21 is then supplied to a drive amplifier 22 where it is amplified to a pair of PWM signals +P22 and xe2x88x92P22 which are 180xc2x0 out of phase with each other, and the PWM signals +P22 and xe2x88x92P22 are supplied to power amplifiers 23A and 23B, respectively. The power amplifiers 23A and 23B are class D amplifiers, and the PWM signals +P22 and xe2x88x92P22 switch a predetermined DC voltage, thereby amplifying the power of the PWM signals +P22 and xe2x88x92P22.
The output terminals of the power amplifiers 23A and 23B are connected to the corresponding ends of a speaker 14 via coils 24A and 24B, respectively, and a filter 25, which is described below. In this case, a capacitor 24C is connected to the output terminals of the coils 24A and 24B. With this configuration, the coils 24A and 24B, and the capacitor 24C form a low-pass filter 24.
Accordingly, the power amplifiers 23A and 23B operate as a single balanced-transformerless (BTL)-type amplifier with respect to the speaker 14. Then, amplified PWM signals +P23 and xe2x88x92P23 are output from the power amplifiers 23A and 23B, respectively, and are integrated by the low-pass filter 24 into the original analog audio signal. The analog audio signal is then supplied to the speaker 14 via the filter 25, which is formed of, for example, a choke coil.
According to the above-configured switching-type power amplifier apparatus 20, the power amplifiers 23A and 23B perform power amplification by turning on or off the voltage and the current in accordance with the PWM signals +P22 and xe2x88x92P22, thereby obtaining a high-power output with high efficiency.
The PWM signals +P23 and xe2x88x92P23 are pulses obtained by PWM-modulating the audio signal. Accordingly, the presence of many spectra can be observed, as shown in FIG. 2, at the frequency positions of the fundamental wave and the higher harmonics, and the spectral components are distributed over a wide frequency range from a few hundred kilohertz to a few hundred megahertz. In the example shown in FIG. 2, the carrier frequency of the PWM signals +P23 and xe2x88x92P23 is 240 kHz. Additionally, the PWM signals +P23 and xe2x88x92P23 are amplified by the power amplifiers 23A and 23B, respectively.
As a result, part of the higher-harmonic components of the PWM signals +P23 and xe2x88x92P23 interferes with the reception of amplitude modulation (AM) broadcasts, FM broadcasts, or television broadcasts, and in some cases, a broadcast of a desired frequency cannot be received. Such higher-harmonic components may adversely influence peripheral electronic circuits or devices.
Accordingly, the low-pass filter 24 is provided for eliminating the interfering noise components (higher-harmonic components). That is, among the noise components contained in the PWM signals +23P and xe2x88x9223P, noise components which are 180xc2x0 out of phase with each other (normal mode noise) cancel each other out.
However, the noise components contained in the PWM signals +P23 and xe2x88x92P23 also include in-phase noise components (common mode noise), which cannot be canceled out in the low-pass filter 24, and are disadvantageously output from the low-pass filter 24. Thus, the filter 25 is provided for the power amplifier apparatus 20 shown in FIG. 1 for eliminating the common mode noise.
Accordingly, in this power amplifier apparatus 20, two filters, such as the filters 24 and 25, are required for eliminating the 180xc2x0-out-of-phase noise components and the in-phase noise components, thereby increasing the cost.
Accordingly, it is an object of the present invention to solve the above-described problem.
In order to achieve the above object, the present invention provides a power amplifier apparatus including:
a pair of power amplifiers, which form a BTL-type amplifier, for performing switching-type power amplification; a transformer including a pair of coils connected in series between the output terminals of the pair of power amplifiers and the corresponding ends of a speaker; and a capacitor connected in parallel with the output terminals of the pair of coils. The pair of coils include a mutual inductance and a leakage inductance.
With this configuration, the 180xc2x0-out-of-phase and in-phase noise components contained in the output of the BTL-type power amplifier can be eliminated by the mutual inductance and the leakage inductance simply by using one transformer and one capacitor. This is effective for space saving, and the cost can be reduced. The number of assembly steps can also be decreased.